Bug 1828259 - Close XUL popup when entering/exiting fullscreen; r=smaug

Differential Revision: https://phabricator.services.mozilla.com/D209355

xpfe/appshell/AppWindow.cpp

@@ -3018,6 +3018,9 @@ void AppWindow::FullscreenChanged(bool aInFullscreen) {
 
 void AppWindow::FinishFullscreenChange(bool aInFullscreen) {
   mFullscreenChangeState = FullscreenChangeState::NotChanging;
+  if (nsXULPopupManager* pm = nsXULPopupManager::GetInstance()) {
+    pm->Rollup({});
+  }
   if (mDocShell) {
     if (nsCOMPtr<nsPIDOMWindowOuter> ourWindow = mDocShell->GetWindow()) {
       ourWindow->FinishFullscreenChange(aInFullscreen);